Combustion apparatus for burning ash-forming liquid fuel



Feb. 16, 1960 M. v. HERBERT 2,924,933

COMBUSTION APPARATUS FOR BURNING ASH-FORMING LIQUID FUEL Filed May 14,1956 3 Sheets-Sheet 1 FIG. I.

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W Maggy/m1 B J'- Feb. 16, 1960 M. v. HERBERT 2,924,938

COMBUSTION APPARATUS FOR BURNING ASH-FORMING LIQUID FUEL I Filed May 14,1956 s Sheets-Sheet 2 FIG Feb. 16, 1960 v, HERBERT 2,924,938

COMBUSTION APPARATUS FOR BURNING ASH-FORMING LIQUID FUEL Filed May 14,1956 5 Sheets-Sheet 3 Lg I makmumm U PEG. 3

COMBUSTION APPARATUS FoR BURNING ASH-FORMING LIQUID FUEL Michael VaughanHerbert, Fleet, England, assignor to Power Jets (Research andDevelopment) Limited, London, England, a company of Great Britain Thisinvention relates to combustion apparatus for burning ash-forming liquidfuel, particularly though not necessarily exclusively fuel oil such asresidual fuel oil' which on combustion yields an ash containingsubstances such as vanadium and sodium compounds which at elevatedtemperatures, e.g., of 650 C. and above, give rise to acceleratedcorrosion of metallic parts with which the ash comes into contact. Suchaccelerated corrosion is a veryserious problem in gas turbines operatingon low grade fuel oils. It is an object of this invention to provide amethod of reducing such corrosion.

It has been found that by controlling combustion in combustion apparatususing fuel oils. of the type indicated above so that the gasesrcsultingfrom the combustion of the fuel include a proportion of unburntcarbon, deposition of ash on and consequent corrosion of the parts uponwhich the ash would normally be deposited can be reduced or eliminated.It would appear that the dry carbon particles tend to absorb the wet andsticky ash, and so much of the ash will be discharged with asubstantially reduced risk of its being deposited on, for example, theturbine blades.

. As pointed out in a co-pending application Serial No. 584,629 in thename of R. l. Hodge filed on the same day as the present application, inorder to eifect combustion control in the'manner indicated above, it isnecessary to control the size of the liquid fuel droplets burnt in theapparatus, since if droplets of a wide range of sizes are burnt, thesmaller ones will be burnt away completely to form ash only, while thelarger ones are only partly burnt. Thus in the case of the smallerdroplets, there will beno unburnt carbon associated with the ashresulting from the combustion of these droplets and this free ash cangive rise to corrosion. On the other hand if combustion is controlled sothat even the smallest droplets are not completely burnt, the exhaustsolids resulting from the combustion of the larger droplets will containa much greater proportion of unburnt carbon, and the resultant loss ofcombustion efiiciency may be unacceptable.

Accordingly the present invention provides combustion apparatus forburning ash-forming liquid fuel comprising a duct leading at one end tothe combustion zone of the apparatus and a fuel injector dischargingfuel into the duct as a generally conical spray of droplets of varioussizes, a fluid stream being discharged from all around the sprayinwardly thereinto to concentrate subcombustion chamber, the fuel to beburnt being in- 2,924,938 Patented Feb. 16', 198i trodueed into andignited in the pre-combustion chamher and continuing to burn while beingcarried around the axis of the vortex chamber in a spiral vortex path.Air may be admitted to mix with and chill the combustioh gases at theoutlet of thevortex chamber.

One embodiment of the invention will now be described by way of examplewith reference to the accompanying drawings, of which:

Figure 1 is a diagrammatic view of a gas turbine plant incorporatingcombustion apparatus of the spiral vortex type shown in longitudinalcross-section.

Figure 2 is a transverse sectional view of the peripheral part of thecombustion apparatus shown in Figure 1.

Figure 3 shows a detail of the arrangement of Figure 2 to a largerscale.

In Figure l a gas turbine plant comprises a compressor 1, a turbine 2driving the compressor through a shaft 3 and also drivinga load such asan alternator 4. The compressor is connected to supply compressed airthrough duct 5. to combustion apparatus of the spiral vortex type andthe turbine is connected to receive hot combustion gases through duct 6from the combustion apparatus whereby'it is driven. a

The combustion apparatus is of the spiral operating on the principlesdescribed in British Patent No. 639,468' and is structurally similar insome respects to the. apparatus described in British Patent Nos.

It comprises an outer casing 11 719,379 and 719,380. having a peripheralwall 114: and two generally frustoconical end walls 11b and 110, and avortex combustion chamber '12, likewise having a peripheral wall 12a,and end walls 12b and 120, mounted in the casing by means of supports 13which maintain the chamber concentric with the casing and restrain itagainst bodily. movement but permit differential thermal expansion. Suchsuppoits are shown in the two last mentioned patents. The combustionchamber has a lining 14 of a refractory material. The end wall 12b ofthe combustion chamber is formed with a central outlet aperture 15 fromwhich extends an axial outlet duct 16 to which is connected the ductv 6for conveying hot combustion gases to the turbine 2, and its other endwall 12c is also formed with a central aperture 13 from whicha tube 19extends axially into the interior of the chamber. Its peripheral wall12a is formed with two diametrically opposite tangentially facinggenerally segment-shaped inlet apertures (one of which is shown at 17)to which are connected generally tangentially extending pre-combustiouchambers 31 as shown in Figure 2, the pre-combustion chambers and thevortex combustion chamber constituting the combustion zone of theapparatus.

The outer casing 11 similarly has a central aperture in its end wall 11band a tubular extension 20 extend ing axially therefrom around theoutlet duct 16. The end of the annular space between the extension 20and duct 16 is closed by an end wall 21, and an air inlet duct 22connected to the duct 5 from the compressor 1 leads laterally into thisspace. The space is further divided radially by an axially extendingtubular wall 23 while the inlet duct 22 is split into two passages ofunequal cross-sectional area by a partition 24, the larger passagecommunicating with the annular space between the extension 20 and thewall 23 and the smaller with the annular space between the wall 23 andduct 16.

The peripheral walls 11a of the casing is further formed with twodiametrically opposite apertures to which are connected generallytangential extensions 32 closed at their ends and constituting aircasings enclosing the precombustion chambers 31 (see Fig. 2).

In operation, the air supply entering the inlet duct 22 is splitbypartition 24 into two streams.

vortex type The smaller stream enters the space between wall 23 and duct16 and 4 enters the latter through rows of holes 16a. Thelarger streamenters the space between the casing 11 and combustion chamber 12, thuscooling the latter, and is further divided, some passing into theaircasings 32 and so into the pre-combustion chambers 31 as combustion air,and the remainder entering the combustion chamber 12 axially throughapertures 18. Butterfly valves 25, 26, separately operable by levers25a, 26a, are provided in the inlet duct 22 for controlling the airstreams on each side of the partition 24. The flow through aperture 18is controlled by a further valve 27 mounted on a threaded rod 28 carriedin the end wall 11c of the casing and operable by hand wheel 29. Byadjustment of these valves, the magnitudes of the various air streamscan be varied relatively to one another.

As shown in Fig. 2, the pre-combustion chamber 31 comprises acircular-section portion 310, an outlet portion 31b of segmental crosssection (conforming to the shape of the inlet 17 to the vortexcombustion chamber as shown in Fig. l) and'a transition portion 31c. Thechamber 31 is enclosed by a similarly shaped outer sheath 33 having aflared end 33a and annularly spaced therefrom by spacers 34 to define anannular passage for cool-, ing air. The sheath has an external flange33b by which it is secured to the peripheral wall 12:; of the vortexcombustion chamber. At the upstream end of the precombustion chamber,there is a constriction of substantially venturi shape having acylindrical throat 35. of

smaller diameter than the circular-section portion 31a.-

The throat'is connected to the. latter by a short frusto-. conical wall36 and a further frustoaconicalwall 37 converges to the upstream end ofthe throat. This last mentioned wall 37 defines with a furtherfrusto-conical wall 38 co-axially within it an annular convergent ,-airinlet passage leading to the constriction, the wall 38 being supportedfrom the wall 37 by axially and radially extending splitters 39. Airflows through this passage from the interior of the extension 32 intothe .pre-cornbustion chamber. V i

Mounted within the inner wall 38 defining the annular air inlet to thepre-combustion chamber 31 is a tube ,or duct 81 open at both ends topermit air to flow from the extension 32 into the pie-combustionchamber. Centrally supported within this tube 81 is a fuel injector 82.Thisinjector is of the known swirl atomizing type having an internalswirl chamber to which fuel is supplied through one or-more tangentialswirl ports and from which the fuel is discharged through an axialdischarge nozzle 82a as a generally conical spray of atomized fuel madeup of droplets of various sizes, the spray being discharged in adirection away from the pre-combustion chamber. The injector is carriedon a streamlined hollow strut 83 extending from the wall of the tube 81,and

liquid fuel, being a residual fuel oil of the type referred to above, issupplied to the injector from fuel tank 45 by, a fuel pump 44 through apipe 84 nested within th strut 83. V

A hollow ring 85 is mounted a short distance upstream ofth'e injector,coaxially therewith and just outside the conical fuel spray, and issupported from the, strut 83 by two diametrically opposite tubes 86.Compressed air is supplied to the interior of the strut 83 from pipe 87and passes through thetubes 86 to the interior of the ring 85 and isdischarged inwardly therefrom into the fuel spray through a large numberof air nozzles 85a symmetrically distributed all around thecircumference thereof. As indicated in the Figure 3, the nozzlesBSa arearranged sothat the air jets emitted. thereby are directed inwardlyandaway from the fuel injector, i. e. downstream with respect to thefuel spray, and are .sub stantialiy normal to the envelope of theconical spray of fuel. It hasbeen' found that in conical fuel spray ofthe type' described there is a naturallyoccurring tendency. for thesmaller fuel droplets emitted by the injector to i be concentratedtowards the axis ofthespray. The air jets from the nozzles 85aaccentuate thistendency as they are able to deflect the smaller dropletstowards the central region of the spray but do not aifect the paths ofthe larger droplets to such a great extent on account of thegreaterinertia of the latter. 7 v I J The smaller droplets which becomeconcentrated towards the axis of the spray are removed through the openend of a collector pipe 88coaxial with and spaced from the discharge endof the injector 82. Thispipe is supported by a spider-89 within the tube81 and extends through the closed end 32a of the extension 32. Bysuitable choice of the relative position and size of the injector 82,ring 85 and collector pipe 88, and of the air and fuel pressures, it ispossible to arrange for. the open end of the collector pipe to receivesubstantially all the fuel droplets of below a predetermined size.

'In operation, the air entering the pre-cornbustion cham- I ber is at apressure higher than atmospheric, having been compressed by compressor1, and so if the other end of the collector pipe 88 is connected to aregion of lower pressure, some of the air will tend to flow down thepipe .and carry away smaller fuel droplets. The pipe 88 is connected tothe inlet of a cyclone separator 47 in which the fuel is separated fromthe air stream, the separated fuel being collected in trap 48 from whichit drains through pipe 49 back to the fuel tank 45. The air is dis-:charged through outlet duct 50 to a region of lower pres annularpassageforpart of the air, tapering towards the constriction. The linerincludes a short cylindrical portion 51a projecting into the throat 35of the constriction. Swirlers 52 in the passage cause the air to flowwith swirl about the axis. of the constriction and the quantity of airand its velocity is so related to the size and configuration of theconstriction that this swirling air on entering the pre-combustion.chamber 33 givesrise to re-circulation along the axis thereof.The-larger fuel droplets are carried into the pre-combustion chamber bythe air flowing through tube 81 and are ignited by an ignition devicesuch as a torch igniter 53, combustion beinginitiated in the stabilizedflame zone. set up in the pre-combustion chamber by. the re-circulation.

The short cylindrical portion 51a of the liner causes the swirlingair toflow along the wall of thethroat of theconstriction, .thus reducing anytendencyfor reverse flow along the wall to occur with consequentdeposition of unburned fuel thereon. This arrangement alsoreduces therisk of the flame blowing back through the layer of air close to thethroat Wall.

Fueloils of the type used in the present apparatus contain v.a-certainproportion of comparativelyvolatile constituents and at least partialcombustion ofthese constituents takes place in the pre-combustionchambers 33. The burning fuel droplets are carried into the vortexcombu'stion chamberll and the less volatile constituents are burn't -asthe fuel droplets are .carried'around the axis of the chamber iri'aspiral vortex path, progressively approa'ching the centraljoutlet 15 .asthey are burnt away. Since the. size of.- the fuel droplets burnt iscontrolled, substantiallyall being above a pre-determined size, theywill all be burnt away to substantially the same extent when they reachthe outlet. Thus by appropriate design of the vortex combustion chamberit may be arranged that thefuel droplets are. only partially. burnt anda pre-d etenminedproportion of unburnt carbon remains assoclatedwith thefuel ash from each fueldroplet when it reaches the outlet. To ensureincomplete combustion and so to obtain a certain proportion of unburntcarbon in the combustion gases discharged through the outlet duct 16,the gases are mixed with and chilled by the air entering through thetube 19 and holes 16a in the outlet duct 16. Combustion is thus stoppedbefore it is complete and the resultant free carbon in the exhaustsolids serve to reduce or eliminate deposition of ash on the turbineblades and elsewhere in the gas turbine as mentioned above. Thischilling air constitutes the dilution air required to reduce thecombustion gas temperature to a value which can be supported by theturbine, the quantity of air entering the pre-combustion chambers 33being sufiicient to support combustion of fuel.

It will be noted that the air in strut 83 serves to protect to someextent the fuel in pipe 84 against overheating.

The pro-combustion chamber is as described in a copending applicationSerial No. 584,577 in the name of J. A. Gardiner filed on the same dayas the present application. The device for introducing the fuel dropletsas described may be used in conjunction with a vaporizing duct leadingto a pre-combustion chamber as described in the above-mentionedco-pending application in the name of R. I. Hodge.

The pre-combustion chamber may, instead of being connected to the inletof a vortex combustion chamber, constitute the upstream end of a tubularflame tube of the knownstraight through flow type. The flame tube willbe designed so that the fuel droplets are not com- If necessary chillingair may be introduced at the flame tube outlet through apertures in theflame tube wall.

The compressed air for the discharge ring 85 may be supplied by a boostpump drawing in air at high pressure from the outlet of the compressor1.

I claim:

l. Combustion apparatus for burning an ash-forming liquid fuelcomprising means defining a combustion zone; a duct leading at one endto said combustion zone; a fuel injector mounted within said duct, saidinjector being of the type which discharges said fuel as a generallyconical spray of droplets of various sizes; means for discharging afluid stream from all around said conical spray inwardly thereinto at avelocity so related to the mass, dimensions and velocity of the fueldroplets as to cause substantially all the smaller fuel droplets ofbelow a predetermined size to be deflected towards the central region ofthe spray; a collector pipe having an open end located in said centralregion of the spray to receive said smaller droplets; means to supplyairto the end of said duct remote from the combustion zone end to carry theremainder of the droplets into said' combustion zone; an igniter forigniting said remainder of the droplets in said combustion zone; saidzone being shaped and dimensioned so that said ignited droplets are onlypartially burnt therein so that a proportion of unburnt carbonassociated with the fuel ashis formed, and having an outlet forcombustion gases.

2. Combustion apparatus according to claim 1 comprising a hollowdischarge ring around the spray formed with at least one nozzle directedto discharge inwardly into said spray and means to supply a fluid tosaid ring.

3. Combustion apparatus according to claim 2 wherein the nozzzle isdirected substantially at right angles to the envelope of the conicalspray.

4. Combustion apparatus according to claim 1 comprising means forcausing an air flow down said collector pipe from said open end.

5. Combustion apparatus according to claim 4 wherein said meanscomprises a fuel separator having an inlet connected to said collectorpipe.

6. Combustion apparatus according to claim 1 having means defining anair entry to said combustion zone and means for swirling the air flowingthrough said entry about the axis thereof.

7. Combustion apparatus according to claim 6 comprising means defining aconstriction at the entry to said combustion zone.

8. Combustion apparatus according to claim 1 wherein said combustionzone is constituted by a vortex combustion chamber defined by two sidewalls, one of which is formed with a central axial outlet, and aperipheral wall formed with a generally tangentially facing inlet, and agenerally tangentially extending pre-combustion chamber leading to saidinlet, the vortex combustion chamber defining a spiral vortex path fromsaid inlet inwardly to i said axial outlet.

9. Combustion apparatus according to claim 8 further comprising meansfor introducing further air into the vortex combustion chamber in theregion of said outlet.

l0. Combustion apparatus for burning an ash-forming liquid fuelcomprising means defining a combustion zone;

a duct leading at one end to said combustion zone; a fuel injectormounted within said duct, said injector being of the type whichdischarges said fuel as a generally conical spray of droplets of varioussizes; means for discharging a fluid stream from all around said conicalspray inwardly thereinto at a velocity so related to the mass,dimensions and velocity of the fuel droplets as to cause substantiallyall the smaller fuel droplets of below a predetermined size to bedeflected towards the central region of the spray; a collector pipehaving an open end located in said central region of the spray toreceive said smaller droplets; means to carry the remainder of thedroplets into said combustion zone; an igniter for igniting saidremainder of the droplets in said combustion zone; said zone beingshaped andvdimensioned so that said ignited droplets are only partiallyburnt therein so that a proportion of unburnt carbon associated with thefuel ash is formed, and having an outlet for combustion gases.

References Cited in the file of this patent FOREIGN PATENTS 1,085,458France Feb. 2, 1955 719,380 Great Britain Dec. 1, 1954

